Optical Time-Domain Reflectometry (OTDR) is a technique for analysing optical fibres as well as other optical components, and is commonly used in the telecommunications industry for analysing breaks in fibre. The technique consists of sending a series of optical pulses along the fibre under test. Light that is backscattered or reflected within the fibre returns back up the fibre and is detected by a photodetector at the point of injection of the optical pulses. The time elapsed from injecting an optical pulse to the time at which a return signal is received provides an indication of the distance to the backscatter or reflection location, because the speed of light in the fibre is known. The technique allows points of high attenuation in the fibre, such as breakages and splice loss, to be detected.
For commonly used OTDR the optical pulse has no coherence requirement. The duration of the optical pulse determines the spatial resolution at which reflection events are measured, that is the spatial resolution is generally limited to the order of the width of the pulse in the fibre. The intensity of the optical pulse determines the dynamic range of the measurement, that is, the ability to determine the location of ever smaller reflection sites.
A related technique uses coherent light pulses in single mode optical fibre. The coherence allows components of the backscattered light to interfere and contribute an intensity variation at the photodetector. The magnitude of this intensity variation depends on the strength with which the light is backscattered and the phase of the light at the point of backscatter. Within the fibre the magnitude and phase of the backscatter vary depending on the position along the length of the fibre. The variation arises from minute variations inherent in the glass of the fibre. External influences or disturbances such as temperature and pressure or the presence of acoustic waves can cause changes in the refractive index of the optical fibre. These changes in refractive index result in a change to the speed of the light pulse and backscattered light along the fibre. The phase of the backscattered light received at the photodetector therefore change as a result of these external influences. Hence, the intensity of the backscattered light also changes under these external influences.
Prior art techniques which use coherent light have found it difficult to determine the location of disturbances or external influences along the length of the fibre. As a result resolution has been limited to the pulse length. To increase resolution the pulse length can be reduced but this often has a consequential effect of reducing the optical power in the pulse. Reduced pulse power reduces the optical power incident on the photodetector, decreasing signal to noise ratio, making it more difficult to perform a useful analysis of disturbances along the fibre.
U.S. Pat. No. 5,194,847 describes an intrusion detection system using coherent light to detect changes in the environment around the fibre that cause perturbations in the optical fibre. The system requires the use of a very coherent light source, for example, having a spectral width of the order of 1-10 kHz. The system is not able to provide information regarding the magnitude of the disturbance acting on the fibre because the change in intensity of light detected by the photodetector does not vary linearly with the magnitude of the disturbance.
WO 2006/048647 describes a technique which uses two partially coherent pulses which interfere following reflection from spaced locations in the fibre. The two pulses allow the detection of localised changes in the refractive index of the fibre. This localisation occurs because the phase change over a localised length of fibre can be measured. Changing the separation between the two pulses allows the length of fibre over which the localisation occurs to be changed. Because the disturbances in the localised length of fibre are generally much less than π, the magnitude of the phase change is now proportional to the magnitude of the disturbance. Therefore, the magnitude of the disturbance along the fibre can be determined.
WO 2008/056143 mentions that the above described methods are limited by the amount of light that can be launched into the fibre by non-linear effects, such as Brillouin scattering. This type of scattering causes the light to be inelastically backscattered converting it to a different wavelength. Brillouin scattering will attenuate a pulse as it travels down the fibre. Although this type of scattering occurs at all optical powers it increases significantly above a threshold. As a result, the narrow spectral width pulse used in U.S. Pat. No. 5,194,847 severely limits the amount of optical power that can be used in a single pulse, and therefore the technique is unable to provide measurements of changes in refractive index to a high sensitivity.
For a light pulse having a spectral width of less than 17 MHz (the Brilluoin Gain Bandwidth) and a wavelength around 1550 nm travelling in single mode fibre made of silica glass, the power threshold is as low as 5 mW. A paper “Polarisation Discrimination in a Phase-Sensitive Optical Time-Domain Reflectometer Intrusion-Sensor System”, Juarez et al, Optics Letters, Vol. 30, No. 24, 15 Dec. 2005, describes an improved method based on U.S. Pat. No. 5,194,847 which uses longer duration, lower power pulses to avoid the problems of Brillouin scattering. However, the proposed pulse length is around 2 μs which limits spatial resolution to around 200 m.
In considering the spectral width of the light pulse WO 2008/056143 proposes light pulses of duration of around 10 ns which have a theoretical spectral width (1/T) of around 0.1 GHz. Pulse widths up to 100 GHz are also considered. By increasing the spectral width the power at any particular wavelength is decreased avoiding Brillouin scattering.
The above described examples can be used in many different environments for detecting very different kinds of disturbances. For example, the system may be used as an intruder detection system where a fibre is laid in the ground around the perimeter of a restricted area. Footsteps in the vicinity of the fibre produce acoustic vibrations which can be detected by temporal changes in the refractive index of the fibre, indicating the presence of an intruder. In another example the fibre can be inserted in a pipeline carrying fluid, such as water, oil, or gas. Cracks in the pipeline or objects hitting the pipe can be detected by the acoustic waves generated by the cracking or hitting event. These two examples both have the problem that although the location of the disturbance along the length of the fibre can be determined, there is no information about from which direction the disturbance occurs. In the first example, the intruder detection system, the direction of the footsteps can be particularly important because instead of indicating an intruder about to cross the perimeter and enter the restricted area, it might indicate a security guard walking around the inside of the perimeter. Thus, it would be advantageous to be able to determine if the footsteps or other disturbance is outside the perimeter or inside the perimeter. Similarly for a detection system in a pipeline, it would be advantageous to determine the direction of origin of the acoustic disturbance to be able to locate the crack easily, or obtain direction information on the hit event.